ROSAT and optical observations of two X-ray transients: MX 0836-42 and GS 0834-430

Two transient sources were detected in hard X-rays by several instruments at the end of 1990. Both sources were observed with Rosat in the all the sky survey and during the pointing phase. The positive detection of the sources in the Rosat pointing observations made possible to derive accurate positions for both sources. Optical images of the Rosat error boxes obtained with the European Southern Observatory new technology telescope are presented. Pulsations are clearly detected from GS 0834-430 with a period of 12.3203 +/- 0.0015s. The pulse profile is roughly sinusoidal with a 13 percent rms variation. The comparison with hard X-ray data and the possible nature of the sources are discussed.